Impedance based sensors are well known, and are commonly used for measuring variables such as temperature, pressure, volume, density, viscosity and the like where the impedance of the sensor is responsive to the particular variable being measured. Such impedance based sensors are also used in metal detectors, proximity sensors, for determining the proximity of one article to another, and the like. However, the impedance of many such sensors is complex, where the sensor includes a significant amount of reactance from capacitive or inductive elements. In such sensors the complex impedance comprises a resistive element, as well as the reactive elements. The reactive elements cause current through the sensor to either lead or lag the voltage. Complex impedance, which commonly is represented by the letter Z, can thus be represented by the equation Z=R+jX where R represented by the letter Z, can thus be represented by the equation Z=R+jX where R represents the resistive element of the impedance, and X is the reactive element. Complex impedance generally varies as a function of frequency, and accordingly, in order to determine the variable being measured, one must determine the frequency range over which measuring of the variable is to take place.
Examples of impedance based sensors in which the impedance is complex are piezo-electric resonators such as bar benders, disc benders, cantilevers, tuning forks, micro-machined membrane and torsion resonators. Such sensors are suitable for use in the measurement of viscosity and density of liquids, see for example a paper entitled “Application of flexural mechanical resonators to high throughput liquid characterization” of L. F. Matsiev, Proceedings of 2000 IEEE International Ultrasonics Symposium, Oct. 22-25, 2000, at page 427-434. Matsiev describes the use of such impedance based sensors in the measurement of density and viscosity of various liquids. The impedance of such sensors varies depending on the liquid, the density or viscosity of which is being measured.
Such impedance based sensors may also be used for measuring density and viscosity of gases, and for measuring many other variables, which have already been discussed above. In general, such impedance based sensors require that a stimulus frequency signal be continuously applied to the sensor in order to analyse a response signal from the sensor. Furthermore, in general it is necessary to be able to sweep the stimulus signal through a range of frequencies. The use of all such impedance based sensors requires the provision of a number of separate elements in order to permit measurement of density or viscosity of liquids to be measured using such sensors.
Firstly, a signal generator is required for generating the stimulus frequency signal for applying to the impedance based sensor. Additionally, the signal generator must be capable of sweeping the stimulus signal through a range of frequencies suitable for making the desired measurement. Secondly, a separate signal conditioning circuit, in general, is required for conditioning the stimulus signal prior to it being applied to the impedance based circuit. Thirdly, a separate signal analysing circuit is required for analysing response signals from the sensor in response to the stimulus signal. Fourthly, in general, a separate signal conditioning circuit is required for conditioning the response signals received from the sensor in response to the stimulus signals, prior to the response signals being analysed in the analysing circuit. After analysis of the response signals, measured values of the variable to be measured are then determined in a separate circuit. This results in many problems, in particular, each such circuit, namely, the signal generator, the signal conditioning circuits and the signal analysing circuit, each introduce errors into the respective signals. Matching of the components of the various separate circuits for minimising mismatch errors is also problematical. Furthermore, the errors introduced into the signals by each separate circuit are cumulative. Another problem with such arrangements is that the signal generator must be set up to provide the stimulus signal continuously at specific frequencies, and additionally, must be capable of being accurately swept through an appropriate range of frequencies, depending on the variable and the liquid which is being measured.
Additionally, in the testing of circuits, such as circuits with complex impedance, and in the testing of transmission lines, such as, for example, transmission lines used in local area networks, telecommunications and the like, both of which include complex impedance, it is necessary to determine characteristics of the impedance of such circuits and transmission lines. This also requires applying a stimulus signal of variable frequency continuously to the impedance circuit to be tested or the transmission line, and simultaneously analysing the response signal received from the impedance circuit or transmission line in response to the stimulus signal. The testing of such impedance circuits and transmission lines also requires the provision of a separate signal generator for generating the stimulus signal, a separate conditioning circuit for conditioning the stimulus signal prior to being applied to the circuit to be tested, a separate conditioning circuit for conditioning the signal received in response to the stimulus signal, and a separate analysing circuit for analysing the response signal received in response to the stimulus signal. By virtue of the fact that separate circuits must be used in determining the impedance characteristic or characteristics of such impedance circuits and transmission lines, similar problems arise in the measuring of the impedance characteristic as already described with reference to impedance based sensors.
There is therefore a need for a measuring circuit and a method for determining a characteristic of the impedance of a complex impedance element such as, for example, an impedance based sensor, a circuit or transmission line with complex impedance for facilitating characterization of the impedance thereof which overcomes these problems.
The present invention is directed towards providing such a measuring circuit and a method.